Electromagnetic field source device with detection of position of secondary coil in relation to multiple primary coils

ABSTRACT

An electromagnetic field source (EFS) for providing electromagnetic energy to a secondary coil, including two or more primary coils that each carry a time-varying current to produce an electromagnetic field, and a controller that selectively provides current to one or more primary coils based on their position with respect to the secondary coil. The secondary coil may be implanted in a human recipient and used to provide power for the operation of a medical device, such as an artificial heart or ventricular assist device. The invention also provides such a secondary coil and EFS, collectively referred to as a transcutaneous energy transfer (TET) device. The primary coils of the EFS or TET may be housed in furniture. For example, they may be housed in a bed mattress or mattress pad on which the recipient rests, or in a blanket for covering the recipient. The controller includes a proximity detector that identifies those primary coils that are closest to the secondary coil, and a current director that, responsive to the proximity detector, selectively directs time-varying currents through the closest primary coils. The controller may also include an orientation detector, coupled to the current director, that determines an orientation of the secondary coil with respect to the closest primary coils. In one implementation, the proximity detector identifies the quantity of closest primary coils utilizing a resonance frequency shift detector that detects a shift in inductance of one or more primary coils due to the proximity of the secondary coil.

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/304,198,filed on May 3, 1999, now U.S. Pat. No. 6,212,430 and titled“Electromagnetic Field Source with Detection of Position of SecondaryCoil in Relation to Multiple Primary Coils.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to energy transfer devices and methodsand, more particularly, to devices and processes for transcutaneousenergy transfer (TET) to a secondary coil implanted in a subject.

2. Related Art

Many medical devices are now designed to be implanted in humans oranimals, including pacemakers, defibrillators, circulatory assistdevices, cardiac replacement devices such as artificial hearts, cochlerimplants, neuromuscular simulators, biosensors, and the like. Since manyof these devices require a source of power, inductively coupledtranscutaneous energy transfer (TET) systems are coming into increasinguse. A TET system may be employed to supplement, replace, or charge animplanted power source, such as a rechargeable battery. Unlike othertypes of power transfer systems, TET systems have an advantage of beingable to provide power to the implanted electrical and/or mechanicaldevice, or recharge the internal power source, without puncturing theskin. Thus, possibilities of infection are reduced and comfort andconvenience are increased.

TET devices include an external primary coil and an implanted secondarycoil, separated by intervening layers of tissue. The two coilsconstitute a transcutaneous transformer. The transformer is designed toinduce alternating current in the subcutaneous secondary coil, typicallyfor transformation to direct current to power the implanted device. TETdevices therefore also typically include an oscillator and otherelectrical circuits for periodically providing appropriate alternatingcurrent to the primary coil. These circuits, referred to for convenienceherein as “TET primary circuits,” receive their power from an externalpower source.

Generally, the non-implanted portions of conventional TET systems areattached externally to the patient, typically by a belt or otherfastener or garment, such that the primary coil of the TET isoperationally aligned with the implanted secondary coil. The TET primarycircuits and external power supply are also generally attached to thepatient's body at or near the site of the attachment of the primarycoil. Such a configuration typically is disadvantageous, however,particularly when the patient is sleeping or resting. For example, if apatient is sleeping on a mattress, the patient would likely beuncomfortable, or restricted in movement, if all or some of the TETprimary circuits and external power supply were attached to the patient.In addition to discomfort or restriction of movement, additionaldisadvantages of such body attachments include possibilities of injuryto the patient or the devices. Movements of the patient may alter theposition of the primary coil so that it is not properly positioned overthe implanted secondary coil to achieve a desired or required transferof power.

To overcome these drawbacks, other conventional approaches require onlythe primary coil be attached to the patient. Wires connect the primarycoil to the TET primary circuits, which, with the power supply, may belocated at a distance from the patient outside of the sleeping orresting surface. However, such an alternative configuration also hassignificant disadvantages. First, the primary coil is still attached tothe patient and therefore subject to the above drawbacks may causediscomfort or restriction of movement. Also, as the patient moves, thewires connecting the externally attached primary coil to the TET primarycircuits may become tangled or entangled with bedding or the patient. Inaddition to being uncomfortable, such tangling may result in dislodgingthe primary coil from its required alignment; it may injure the patient,such as by restricting blood or oxygen supply; or it may interfere withtubes or other devices attached to the patient.

SUMMARY

To overcome the above and other drawbacks to conventional systems, thepresent invention provides an electromagnetic field source (EFS) forproviding electromagnetic energy to a secondary coil. In one embodiment,the EFS includes two or more primary coils that each carry atime-varying current to produce an electromagnetic field. The EFS alsoincludes a controller that selectively provides current to one or moreof the primary coils based on their position relative to the secondarycoil. The controller may be implemented in electrical circuits,software, firmware, or any combination thereof.

In another embodiment, the invention provides a transcutaneous energytransmission (TET) device including a secondary coil implanted in ahuman being. In this embodiment, the secondary coil is used to providepower for the operation of an implanted medical device, such as anartificial heart or ventricular assist device. In some implementations,the primary coils are housed in furniture, such as a bed mattress. Also,the primary coils may be housed in bed covering, such as a blanket ormattress pad.

In certain embodiments the controller includes a proximity detector thatidentifies a quantity of primary coils that are closest to the secondarycoil, referred herein to as the “closest” primary coils. A currentdirector that is responsive to the proximity detector is also includedin the controller. The current director selectively directs time-varyingcurrents through the closest primary coils.

One advantage of a TET in accordance with certain aspects of the presentinvention is that there are no wires connecting the subject of theimplanted device to external components, such as a power supply or otherelectrical circuits. Rather, a recipient of the implanted device mayrest on furniture that houses the primary coils, and those primary coilsthat are closest to the implanted secondary coil may be energized. Thus,the recipient is uninhibited with respect to movement on or in thefurniture, such as a bed, couch, or chair, and is provided with a morecomfortable resting environment. Also, serious disadvantages of knownsystems, such as becoming entangled with a wire, or of dislodging aprimary coil, are avoided by the present invention.

Another advantage is the portability of implementations such as those inwhich the primary coils are housed in a bed covering or a mattress pad.Thus, a recipient may pack such housing, together with the controllerand power supply, in suitcases or similar containers for traveling.Similarly, a hospital mattress may readily be converted to includeportions of a TET by covering it with a blanket or mattress padcontaining primary coils in accordance with embodiments of the presentinvention.

Advantageously, the primary coils may be disposed over substantially allof the top surface of the mattress, or throughout the bed covering ormattress pad. Thus, in such embodiments, if the recipient shiftsposition on the mattress, there will be one or more primary coilslocated close to the implanted secondary coil. In some implementations,the primary coils may be positioned in generally even rows and columnswith respect to the top surface of the mattress. In otherimplementations, they may be positioned generally in hexagonalarrangements. It will be understood to those of ordinary skill thatthere are many possible configurations that provide primary coils overthe entire surface upon which the recipient is resting or reclining.

Also advantageously the controller of the EFS or TET may determine theapproximate distance between the primary coils and the secondary coil,and adjust the amount of current to the closest primary coilsaccordingly. In particular, a proximity detector may be included todetermine an approximate distance between one or more of the closestprimary coils and the secondary coil. In embodiments that include such aproximity detector, if that distance is greater than a nominal thresholdvalue, current director may increase the currents through selected onesof the closest primary coils. For example, if the recipient is sleepingon a pillow or is otherwise raised above the mattress, the distance fromthe implanted secondary coil to the primary coils in the mattress may begreater than normal (i.e., greater than when the recipient is sleepingdirectly on the mattress). By increasing the amount of current directedto the closest primary coils, the electromagnetic fields of the closestprimary coils are increased to reach the secondary coil so that it mayprovide power to the medical device or to an energy storage device. Inone embodiment of the EFS or TET, the proximity detector identifies apredetermined number of the closest primary coils. Alternatively, thequantity of closest primary coils may be identified by the proximitydetector based on the size of the secondary coil.

The primary coils may be disposed in their housings in accordance with awide variety of geometric schemes or arrangements. For example, theprimary coils may be disposed in a single plane, or in two or moreparallel planes. It should be understood that the specification hereinof this, and other, geometric arrangements may be approximate. Forexample, it generally is not required that the primary coils be disposedprecisely in a single or parallel planes, in precisely a square pattern,and so on. Rather, they may be approximately so disposed. Similarly, asecond-layer primary coil need not be precisely aligned with first-layerprimary coils, as described below.

In one embodiment in which a configuration of two parallel planes isused, dead zones of electromagnetic fields generated by one or moreprimary coils of a first plane are encompassed by electromagnetic fieldsgenerated by primary coils in a second plane that is parallel to thefirst plane. The term “dead zone” is used herein to refer to a space inwhich an electromagnetic field generated by a primary coil is noteffective in energizing a secondary coil disposed in that space. Thisterm is further explained, and is illustrated, below. The word“energize,” and its grammatical variants, is used herein to refer to theprovision of current to a primary coil so that it produces anelectromagnetic field. The word “encompassed” is used herein in thiscontext to refer to the covering of the dead zone by the electromagneticfield of a second layer of primary coils, such that the electromagneticfield is effective in energizing the secondary coil disposed in the deadzone of a first layer of primary coils.

In some embodiments having primary coils disposed in two parallelplanes, a first plane includes two or more mutually adjacent first-planeprimary coils. The term “mutually adjacent” means that each of two ormore primary coils in a group is adjacent to each of the other primarycoils in that group. Examples of such arrangements are the placement ofthe centers of the primary coils at the comers of a rectangular ortriangular shape in the first plane. In such embodiments, a second planeis provided that has at least one second-plane primary coil (not to beconfused with a secondary coil). These coils are positioned so that theprojection of a magnetic center of the second-plane primary coil on thefirst plane is approximately equidistant from magnetic centers of eachof the two or more mutually adjacent first-plane primary coils. Forexample, if first-plane square primary coils are positioned so thattheir comers are adjacent to each other, then a second-plane primarycoil is placed in the second plane such that the projection of itsmagnetic center onto the first plane aligns with the comers of thefirst-plane square. The term “magnetic center” of a coil is used hereinto mean the geometric center of iso-magnetic contours representing themagnetic field generated by the coil. In a particular implementation ofsuch an arrangement, the first plane has four mutually adjacent primarycoils positioned in a roughly square arrangement. The second plane hasone primary coil positioned so that the projection of its geometriccenter on the first plane is approximately centrally located among thefour first-plane primary coils.

In some embodiments, the EFS or TET also includes an orientationdetector, coupled to the current director, that determines theorientation of the plane of the secondary coil with respect to theplanes of the closest primary coils. In some implementations, theorientation detector is electrically coupled to the primary coils anddetermines the orientation of the plane of the secondary coil utilizinga resonance frequency shift detector. The detector compares shifts ininductance of two or more primary coils due to the proximity of thesecondary coil. In other implementations, the orientation detector maydetermine the plane of the secondary coil utilizing an optical sensor, amechanical sensor, electromagnetic transmission, or any combination ofthese or other sensors now or later developed.

In some embodiments, if the orientation detector determines that thesecondary coil is disposed in a plane predominantly parallel to theplanes of the closest primary coils, the current director directstime-varying currents to flow through the closest primary coils so thateach current flows in the same direction. In one implementation of suchan embodiment, the quantity of closest primary coils may be simple onecoil. In particular, there may be a single closest primary coil if theproximity detector determines that the secondary coil is proximate to anelectromagnetic field of the one closest primary coil, not including adead zone. In another implementation, the quantity of closest primarycoils is two or more when all of the primary coils are disposed in thesame plane and the proximity detector determines that the secondary coilis proximate to a dead zone of the one closest primary coil.

If the orientation detector determines that the secondary coil isdisposed in a plane predominantly perpendicular to the planes of theclosest primary coils, the current director directs time-varyingcurrents to flow through the closest primary coils so that a current ineach closest primary coil flows in a direction opposite to a directionof a current in an adjacent closest primary coil. In one implementationin which the secondary coil is perpendicularly positioned, the quantityof closest primary coils is two.

The EFS may also include a power supply that is coupled to thecontroller and that provides current to the primary coils. It should beunderstood that the power supply may not be directly coupled to theprimary coils, but that intermediary components, such as a regulator,may be present. Alternatively, the regulator or other components may beincluded in the power supply.

In further embodiments, the invention provides a method for providingelectromagnetic energy to a secondary coil. The method includesdisposing primary coils, each constructed and arranged to carry atime-varying current to produce an electromagnetic field, andselectively providing current to the primary coils based on theirposition with respect to the secondary coil. In some implementations,such selection includes identifying a quantity of the primary coils thatare closest to the secondary coil, and selectively directingtime-varying currents through the closest primary coils.

In other embodiments, the invention is a cardiac-assist device includingpumping means and a transcutaneous energy transmission (TET) device. TheTET includes a secondary coil implanted in a subject, primary coils thatcarry a time-varying current to produce an electromagnetic field, and acontroller that selectively provides current to one or more of theprimary coils based on their position with respect to the secondarycoil. The pumping means may include a total artificial heart or aventricular assist device. More generally, the invention provides insome embodiments an organ-assist device including such a TET and aninternally implanted organ-assist component.

In yet further embodiments, the invention is an article of furniturehaving embedded in it two or more primary coils that each carry atime-varying current to produce an electromagnetic field. The furniturealso includes a controller that selectively provides current to one ormore of the primary coils based on their position with respect to thesecondary coil.

Further features and advantages of the present invention as well as thestructure and operation of various embodiments of the present inventionare described in detail below with reference to the accompanyingdrawings. In the drawings, like reference numerals indicate identical orfunctionally similar elements. Additionally, the left-most one or twodigits of a reference numeral identifies the drawing. in which thereference numeral first appears.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further advantages of the invention will be more clearlyappreciated from the following detailed description when taken inconjunction with the accompanying drawings. In the drawings likereference numerals indicate like structures or method steps, in whichthe leftmost one or two digits of a reference numeral indicates thenumber of the figure in which the referenced element first appears (forexample, the element 210 appears first in FIG. 2, the element 1010 firstappears in FIG. 10). In the figures:

FIG. 1 is a schematic and functional block diagram of an electromagneticfield source in accordance with one embodiment of the present invention;

FIG. 2 is a schematic cross-sectional top view of a secondary coil andassociated components, including an artificial heart, implanted in arecipient reclining on a mattress having embedded therein the electronicfield source of FIG. 1;

FIG. 3 is a functional block diagram of one embodiment of a currentdirector of one embodiment of a controller of the electromagnetic fieldsource of FIG. 1;

FIG. 4A is a schematic top view of four primary coils arranged in agenerally square-shape pattern and having the same direction of currentflow, such as may be employed in an exemplary embodiment of theelectromagnetic field source of FIG. 1;

FIG. 4B is a schematic top view of one embodiment of a primary coil thatgenerates a magnetic field generally equivalent to the magnetic fieldsgenerated by the primary coils of FIG. 4A;

FIG. 4C is a schematic top view of eight primary coils arranged in twoadjacent square-shape patterns, the coils of one pattern having oppositedirections of current flow from the coils of the other pattern, such asmay be employed in an exemplary embodiment of the electromagnetic fieldsource of FIG. 1;

FIG. 4D is a schematic top view of one embodiment of two adjacentprimary coils that generate a combined magnetic field generallyequivalent to the magnetic fields generated by the primary coils of FIG.4C;

FIG. 5A is a schematic top plan view of a single primary coil such asmay be employed in an exemplary embodiment of the electromagnetic fieldsource of FIG. 1;

FIG. 5B is a schematic cross-sectional side view of the primary coil ofFIG. 5A, showing magnetic fields produced by the primary coil;

FIG. 6A is a schematic top plan view of two adjacent primary coils,having current flows in the same direction, such as may be employed inan exemplary embodiment of the electromagnetic field source of FIG. 1;

FIG. 6B is a schematic cross-sectional side view of the primary coils ofFIG. 6A, showing magnetic fields produced by the primary coils;

FIG. 7A is a schematic top plan view of two adjacent primary coils,having current flows in opposite directions, such as may be employed inan exemplary embodiment of the electromagnetic field source of FIG. 1;

FIG. 7B is a schematic cross-sectional side view of the primary coils ofFIG. 7A, showing magnetic fields produced by the primary coils;

FIG. 8A is a schematic cross-sectional top view of two horizontallydisposed primary coils in the same plane such as may be employed in anexemplary embodiment of the electromagnetic field source of FIG. 1,including a mattress in which the coils are embedded;

FIG. 8B is a schematic cross-sectional side view of the primary coilsand mattress of FIG. 8A;

FIG. 8C is a schematic cross-sectional side view of two primary coilsdisposed at opposing acute angles to the horizontal, such as may beemployed in an exemplary embodiment of the electromagnetic field sourceof FIG. 1, including a mattress in which the coils are embedded;

FIG. 8D is a schematic cross-sectional side view of one horizontallydisposed primary coil and two vertically disposed primary coils atopposing ends of the horizontal coil, such as may be employed in anexemplary embodiment of the electromagnetic field source of FIG. 1,including a mattress in which the coils are embedded;

FIG. 9A is a schematic cross-sectional top view of three horizontallydisposed primary coils, two of which are in the same plane and the thirdof which is in a parallel plane, such as may be employed in an exemplaryembodiment of the electromagnetic field source of FIG. 1, including amattress in which the coils are embedded;

FIG. 9B is a schematic cross-sectional side view of the primary coilsand mattress of FIG. 9A; and

FIG. 10 is a schematic cross-sectional top view of a two-plane,interlaced, array of horizontally disposed primary coils such as may beemployed in an exemplary embodiment of the electromagnetic field sourceof FIG. 1, including a mattress in which the coils are embedded.

DETAILED DESCRIPTION

I. Introduction

The attributes of the present invention and its underlying method andarchitecture will now be described in greater detail in reference to oneembodiment of the invention, referred to as an electromagnetic fieldsource (EFS) 100, aspects of which are represented in FIGS. 1 through10. In this detailed description, references are made to variousfunctional modules of the present invention that may be implementedusing software, hardware including electronic circuits, firmware, or anycombination thereof. Some of these implementations may also include amicroprocessor (not shown) to execute the software or firmware inaccordance with techniques well known to those skilled in the relevantarts. Such a microprocessor may be one of a variety of known types, or amicroprocessor to be developed in the future.

FIG. 1 is a schematic and functional block diagram of EFS 100 suitablefor implementation in a transcutaneous energy transfer (TET) system. EFS100 primarily includes primary coils 110A and 110B (collectively andgenerally referred to as primary coils 110) and a controller 120. Asshown in FIG. 1 EFS 100 is connected to TET primary circuits 140 and apower supply 130. Primary. coils 110 generate electromagnetic fields forproviding energy to a secondary coil that, in this exemplary embodiment,is implanted in a human recipient. Controller 120 selectively providescurrent to one or more of primary coils 110 based on a number of factorssuch as the relative position and orientation of the primary andsecondary coils, the configuration of the coils, the power to betransferred, the magnitude of the current, among others. Such factorsare determined by the application of the invention and the manner inwhich the invention is implemented, as described in detail below.

A particular implementation of EFS 100 for use as part of atranscutaneous energy transfer (TET) device for powering an artificialheart is shown in FIG. 2. FIG. 2 is a schematic drawing of EFS 100embedded in a mattress 210. Also schematically shown is a human implantrecipient 205 reclining on mattress 210. The implanted componentsinclude a total artificial heart 215, an internal battery 220, asecondary coil 230, internal electronics 240, and connecting wires 250.Embedded in the mattress are primary coils 110A and 110B (shown in analternative, oval, configuration), controller 120, TET primary circuits140, and power supply 130.

II. Primary Coils

As shown in FIG. 1, EFS 100 includes an exemplary configuration of twoprimary coils, primary coils 110A and 110B. It will be understood thatmany other configurations are possible in accordance with the presentinvention. Specifically, any number of primary coils may be included inother configurations. The primary coils are not limited to the squareshape of coils 110 shown in FIG. 1 or the oval shape shown in FIG. 2.Coils 110 may be any shape in two or three dimensions, and the coils maybe arranged in any pattern rather than be limited to the row patternshown. The direction of current flow may be in the clockwise (CW)direction, rather than in the counter-clockwise (CCW) direction shownwith respect to coils 110. Also, any number of loops may be included ineach coil, for ease of illustration, one winding or loop is shown in theillustrative examples of coils 110.

As is well known to those skilled in the relevant art, the magneticfield (B) that is generated by a coil having “N” loops and acharacteristic radius of “r” is given by equation $\begin{matrix}{{B = \frac{\mu_{o}N\quad i}{2r}},} & (1)\end{matrix}$

in which μ_(o) is the permeability constant for free space (equal to 4πtimes 10⁻⁷ weber per ampere·meter), i is the current through the primarycoil in amperes, r is expressed in meters, and B is therefore expressedin weber per square meter.

As also is well known, the magnetic field generated by a larger coil maybe generated by a magnetically equivalent configuration of smallercoils. FIGS. 4A-4D provide illustrative examples. In FIG. 4A, currentsin coils 410A-D (collectively referred to as coils 410) are shown asflowing in a clockwise direction. The coils 410 are roughly squareshape, and are positioned at the corners of a square pattern. It will beunderstood that these current flows show an instantaneous or steadystate direction, and that the direction may be reversed. It is assumedfor illustrative purposes that, when a current flow is reversed, it isreversed for all energized primary coils in the same manner. Thus, forexample, a comparison between two adjoining primary coils in whichcurrents flow in opposite directions generally is applicable both tosteady state and alternating current conditions. For clarity andconvenience, references hereafter to the direction of current flow willbe made with respect to an instantaneous or steady state direction.Alternatively, the direction of current flow may be considered to be thesense of direction of the winding of the coil.

As will be evident to those skilled in the relevant art, some componentsof the magnetic fields of coils 410 tend to cancel each other out.Specifically, those components generated by currents flowing in oppositedirections, such as the loop-segment pairs 411 and 412, or 413 and 414,i.e., those in the interior of the square pattern, produce generallyopposing magnetic fields. In contrast, magnetic fields generated by theloop-segments on the exterior of the square pattern, such as segments415 and 416, are not opposed by magnetic fields generated by other loopsegments in the pattern. Thus, the combination of magnetic fieldsgenerated by primary coils 410 is generally equivalent to the magneticfield generated by primary coil 420 of FIG. 4B.

Similarly, the combined magnetic field generated by primary coils 430A-Hhas a shape that is generally equivalent to the combined magnetic fieldsof primary coils 440. Specifically, the square pattern of coils 430A-D,having current flowing in a clockwise direction, produces a magneticfield generally equivalent to that produced by coil 440A, which also hasclockwise current flow. The square pattern of coils 430E-H, having acounter-clockwise current flow, produces a magnetic field generallyequivalent to that produced by coil 440B. Thus, the configuration ofFIG. 4C is generally equivalent to that of FIG. 4D in terms of themagnetic fields produced by the coils of those figures.

FIGS. 5A and 5B provide greater detail with respect to the magneticfields produced by a single primary coil such as coil 420 of FIG. 4B.FIG. 5A is a schematic top plan view of a single primary coil, coil 510,disposed in a plane 520. It will be assumed for illustrative purposesthat plane 520 is horizontal. For example, it may be a plane in ahorizontally disposed mattress, such as mattress 210. Current flowsthrough coil 510 in a counter-clockwise direction. FIG. 5B is across-sectional side view along cross-section line 5B—5B through loopsegments 511 and 512. The notations “O” and “X” in the cross section ofplane 520 respectively represent current flow out of and into thedrawing page through loop segments 511 and 512. Iso-magnetic contours551 and 552 respectively represent the magnetic fields generated bycurrent flowing through loop segments 511 and 512 in the describeddirections. The direction and magnitude of the magnetic fields areschematically represented by arrows 561-568. For illustrative purposes,relatively weak magnetic fields are represented by small arrows, andlarge arrows are used to represent stronger magnetic fields. It shouldbe appreciated that such diagrammatic representations arerepresentational only and are not intended to be precise with respect totheir size, shape or orientation.

Also shown in FIG. 5B, but omitted from FIG. 5A for clarity, is a secondhorizontal plane 530 disposed above plane 520. It is assumed forillustrative purposes that a secondary coil 230 implanted in recipient205 is disposed within plane 530 at a position represented in phantomoutline by slot 540A. As shown, position 540A is located so that itsprojection on plane 520 is generally within the interior of coil 510.

Components of the magnetic fields generated by loop segments 511 and 512generally reinforce each other in certain areas, such as the interiorarea of coil 510. These reinforced magnetic field components arerepresented by large arrows 564 and 565 that are directed upward fromplane 520 toward plane 530. More specifically, magnetic fieldsrepresented by exemplary iso-magnetic contour lines 551 and 552 areoriented in the same direction (upward toward plane 530), and thus arereinforcing, in the regions of arrows 564 and 565. Elsewhere, such as inthe regions of small arrows 561 and 566, there is no such reinforcement.

If secondary coil 230 is in position 540A, i.e., in a plane parallel tothat of primary coil 510 and generally vertically aligned with theinterior of coil 510, then, as is well known by those skilled in therelevant art, primary coil 510 induces a current to flow in secondarycoil 230. The magnetic field that induces this current flow isrepresented by arrows 564 and 565. In contrast, if secondary coil 230 isdisposed in plane 530 at a position generally outside of the interior ofcoil 510, such as positions 540B or 540C, then primary coil 510generally does not induce a current to flow in secondary coil 230.

More specifically, the magnetic fields acting on secondary coil 230 whenin position 540B or 540C generally are in the same or parallel plane asthe coils, and thus, as is well known, induce weak or no current withthe coils. In addition, even if secondary coil 230 were perpendicular tothe direction of the magnetic field, those fields outside the perimeterof the primary coil (such as are represented by arrows 561 or 566)generally are weaker than those in the interior (such as represented byarrow 564). For convenience, such areas in the magnetic fields; i.e.,those that induce weak or no currents in secondary coils located inthose areas, are referred to hereafter as “dead zones.”

FIGS. 6A and 6B provide greater detail with respect to the magneticfields produced by two primary coils in proximity to each other, such ascoils 110 shown in FIG. 1. FIG. 6A is a schematic top plan view ofprimary coils 610A and 610B (coils 610), disposed in a horizontal plane620. Current flows through both of coils 610 in the same direction,i.e., counter-clockwise in this example. FIG. 6B is a cross-sectionalside view of coils 610 in FIG. 6A taken along cross-section line 6B—6B.Iso-magnetic contours 651-654 respectively represent the magnetic fieldsgenerated by current flowing through loop segments 611-614 in theindicated directions.

As noted, the magnetic fields in the interior of the coils tend toreinforce, and thus produce combined magnetic fields such as arerepresented by large arrows 661 and 662 (with respect to coil 610A) and663 and 664 (with respect to coil 610B). Also, between coils 610A and610B the magnetic fields generated by loop segments 612 and 613 tend toreinforce each other in the upward direction, as represented by largearrow 665.

A second horizontal plane 630 is shown above plane 620 in FIG. 6B, butis omitted from FIG. 6A for clarity. As with respect to FIG. 5B, it isassumed for illustrative purposes in FIG. 6B that secondary coil 230 maybe positioned in various locations in plane 630, as indicated byrepresentative regions 640A-640C shown in phantom outline. Ahorizontally-disposed secondary coil 230 is activated when located inany region 640 due to then orthogonal intersection with thevertically-oriented magnetic fields generated by primary coils. The word“activated” means in this context that one or more primary coils haveproduced a magnetic field of any strength, and in an orientation,sufficient to induce an operative current in secondary coil 230. Theword “operative” means that the current is sufficient to enablesecondary coil 230 to perform its function, such as powering circuitsrelated to artificial heart 215 or recharging internal battery 220.

Another case is now considered in which current is flowing in oppositedirections through two adjacent primary coils. FIG. 7A is a schematictop plan view of primary coils 710A and 710B (coils 710), disposed in ahorizontal plane 720. Current flows through coil 710A in acounter-clockwise direction, and current flows through coil 710B in aclockwise direction. FIG. 7B is a cross-sectional side view taken alongcross-section line 7B—7B through loop segments 711 and 712 of coil 710Aand loop segments 713 and 714 of coil 710B. Current flows out of thedrawing page through loop segments 711 and 714, and into the drawingpage through loop segments 712 and 713. Iso-magnetic contours 751 and753 respectively represent the magnetic fields generated by currentflowing through loop segments 711 and 714. Iso-magnetic contour 752represents the combined magnetic field generated by current flowingthrough loop segments 712 and 713.

Large arrows 761 and 762 represent the relatively strong magnetic fieldsgenerated in a vertical direction in the interior of primary coil 710Adue to reinforcement, as described above. Similarly, large arrows 763and 764 represent the relatively strong magnetic fields generated in avertical direction in the interior of primary coil 710B. In contrast, inaccordance with effects well known by those skilled in the relevant art,the magnetic fields generated by loop segments 712 and 713 tend togenerate a relatively strong magnetic field in a horizontal direction.This horizontal magnetic field between coils 710A and 710B isrepresented by large arrows 765 and 766. Also, non-reinforced magneticfields are generated in a generally horizontal orientation by loopsegments 711 and 714, as indicated by small arrows 767 and 768.

A representative series of five vertical positional slots 730A-E(positions 730) are shown above plane 710 in FIG. 7B, but are omittedfrom FIG. 7A for clarity. It is assumed for illustrative purposes thatsecondary coil 230 may be positioned in any position 730 ortherebetween. If secondary coil 230 is positioned in positions 730B or730D, i.e., vertically oriented approximately over the middle of coils710A and 710B, respectively, coil 230 generally will not be activated.Lack of activation is due to the fact that the vertical orientation ofsecondary coil 230 coincides with the vertical orientations of themagnetic fields (arrows 761 and 763) in these locations. If secondarycoil 230 is positioned in position 730C approximately between coils 710Aand 71OB, it generally is activated because the vertical orientation ofsecondary coil 230 is perpendicular to the horizontal orientation of themagnetic field (arrow 765) at this location.

Similarly, secondary coil 230 generally is activated if it is positionedin positions 730A or 730E, that is, approximately over the exterior loopsegments 711 and 714 of coils 710A and 710B, respectively. The word“exterior” is used in this context to refer to loop segments of anenergized primary coil located generally opposite to loop segments ofthe same coil that are close to loop segments of another energizedprimary coil. Activation is attained because the vertical orientation ofsecondary coil 230 is perpendicular to the horizontal magnetic field.

However, because the magnetic fields represented by arrows 767 and 768are not reinforced, it may be necessary to increase the strength ofthese fields in order to sufficiently activate secondary coil 230 tomeet desired performance standards. As indicated by equation number one,one way to increase the magnetic fields represented by arrows 767 or 768is to increase the current through primary coils 710A or 710B,respectively. For example, if coils 710 are located at the edge, thenthe currents may be increased. Also, the number of loops in the primarycoils may be increased based on a determination of the strength thatthese weaker magnetic fields must exhibit in order to activate secondarycoil 230.

Aspects of the operation and arrangement of primary coils of EFS 100 arenow further described with respect to an illustrative example. It isassumed in this example that recipient 205 may move to any position onmattress 210, and assume any orientation between horizontal (i.e. lyingon the back or stomach) and vertical (i.e., lying on a side). Also,recipient 205 may be positioned immediately above mattress 210, or at agreater distance from it, as when lying on pillows or sitting in bed.Thus, secondary coil 230 may be aligned above any portion of mattress210, may be located either on or at a distance from mattress 210, andmay be disposed in any orientation with respect to the plane of mattress210.

In accordance with one embodiment, a small number of relatively largeprimary coils, such as coils 110A and 110B as shown in FIG. 2, may beused. FIGS. 8A-8D show three illustrative configurations employing asmall number of large primary coils in a horizontally disposed mattress800.

FIG. 8A is a schematic cross-sectional top view of mattress 800including primary coils 810A and 810B (coils 810). FIG. 8B is across-sectional side view of the configuration of FIG. 8A alongcross-section line 8B—8B. The configuration of FIGS. 8A and 8B issimilar to that of FIG. 1 with respect to the location of primary coils110. As shown in FIG. 8B, coils 810 are disposed approximately in thesame plane.

It may first be assumed that secondary coil 230 is positioned aboveeither primary coil 810A or 810B (i.e., so that the vertical projectionof secondary coil 230 falls approximately within one of coils 810). Itis also assumed that secondary coil 230 is horizontally disposed; i.e.,it is in approximately the same plane as primary coils 810. In thiscase, secondary coil 230 may be activated by providing current, ineither direction, to whichever of coils 810 secondary coil 230 is above.This primary coil is referred to for convenience as the “closest”primary coil, meaning that it is the “closest” of the primary coils tothe secondary coil. Hereafter, “closest” similarly will be used to meanthe primary coil, or group of primary coils, that are closer to thesecondary coil than any other primary coils.

The closeness, or proximity, of coils to each other may be defined anddetermined in different ways, depending, for example, on the type ofproximity sensor that is used. Thus, for a resonance frequency shiftsensor, such as is described below with respect to the operations ofproximity detector 126, closeness generally relates to magneticproximity in the sense that coils are closest if their mutual inductanceis strongest. If other kinds of sensors are used, such as optical ormechanical sensors, closeness generally relates to the physicalproximity of the coils.

It is now assumed for illustrative purposes that secondary coil 230 ispositioned above and approximately between coils 810. In this case,secondary coil 230 may be activated by applying currents to coils 810Aand 810B in the same direction. The magnetic fields generated by coils810 thus generally are as shown in FIG. 6B with respect to coils 610.That is, a strong, vertically oriented, magnetic field (such as shown byarrow 665 of FIG. 6) perpendicularly intersects horizontally positionedsecondary coil 230.

Secondary coil 230 is now assumed to be disposed in a verticalorientation and located approximately between primary coils 810. In thiscase, secondary coil 230 may be activated by applying currents to coils810A and 810B in opposite directions. The magnetic fields generated bycoils 810 thus generally are as shown in FIG. 7B with respect to coils710. That is, a strong, horizontally oriented, magnetic field (such asshown by arrow 765 of FIG. 7) perpendicularly intersects verticallypositioned secondary coil 230. Similarly, if secondary coil 230 ispositioned generally above an exterior loop segment of coils 810, suchas loop segments 801 or 802, it will be intersected by a magnetic fieldperpendicular to it, although this field generally will be weaker thanthe one between the coils. However, as noted above with respect tosimilar fields 767 and 768 of FIG. 7, secondary coil 230 may beactivated by increasing the current through the closest coil so that themagnetic field is increased in strength, or by varying other parameterssuch as the number of loops in the primary coils.

Finally, the case is considered in which secondary coil 230 is disposedin a vertical orientation and located approximately above the middle ofone of primary coils 810. In this case, it generally is not possible toactivate secondary coil 230 because it is in approximately the sameplane as the magnetic field generated by the closest primary coil. Thissituation is shown in FIGS. 6B and 7B with respect to verticallyoriented magnetic field arrows 661, 663, 761, and 763. Thus, theconfiguration of FIG. 8B; i.e., in which the primary coils are in thesame horizontal plane, generally is not appropriate if secondary coil230 may be in a predominantly vertical orientation and it is requiredthat it may be activated at every location over horizontal mattress 800.

However, the configuration of FIG. 8B may nonetheless be employed in avariety of circumstances. For example, secondary coil 230 may beconstrained to remain predominantly in a horizontal position, i.e., in aplane predominantly parallel to the plane of the primary coils.Alternatively, more than one secondary coil may be used; for example,two secondary coils that are perpendicularly oriented with respect toeach other may be employed.

To illustrate a more general case, it is now assumed that the primarycoils of EFS 100 must be positioned so that one or more of them mayactivate a single secondary coil 230 irrespective of the location ororientation of recipient 205 with respect to mattress 210. Forconvenience, this illustrative condition is referred to as a requirementfor “full coverage.” It will be understood that, in alternativeembodiments, full coverage need not be required. For example, in someapplications it may be provided that a secondary coil need not beactivated if it is located at certain positions on or over the housingof the primary coils, or, as just noted, is disposed in particularorientations.

FIGS. 8C, 8D, 9A, and 9B show illustrative configurations that may beused to achieve full coverage. It will be understood that manyalternative configurations are possible. In FIGS. 8C, 8D, 9A, and 9B, incontrast with the configuration of FIGS. 8A and 8B, the primary coils ofEFS 100 are not disposed in the same plane. FIG. 8C is an illustrativeexample of two primary coils 820A and 820B (coils 820) that are similarto primary coils 810 except that they are disposed on separate,intersecting, planes. In particular, coil 820A forms an acute angle fromnear the center of the bottom surface 806 of mattress 800 upwardstowards side 801 and top surface 805 of mattress 800. Primary coil 820Bsimilarly forms an acute angle from near the center of the bottomsurface 806 upwards towards an opposite side 802 of mattress 800. Thus,primary coils 820 form a roughly “V” shape in cross section.

Assuming again for convenience of reference that mattress 800 ishorizontally oriented, both of coils 820 may generate magnetic fieldsthat are perpendicular to their planes and thus have both horizontal andvertical components. Therefore, if secondary coil 230 is verticallydisposed at any point above the interiors of either of coils 820, ahorizontal component of the magnetic field of the closest of primarycoils 820 may be generated in order to activate secondary coil 230. Asis evident, the amount of current required to activate secondary coil230 depends, among other things, on the angles of coils 820 with respectto the horizontal, and on the sizes and numbers of loops of the coils.Also because of the presence of both horizontal and vertical components,and for the reasons discussed above with respect to inter-coil magneticfields as illustrated in FIGS. 6B, 7B, and 8B, a vertically orientedsecondary coil 230 also is activated if it is located between coils 820.

Similarly, because coils 820 generate magnetic fields having verticalcomponents, secondary coil 230 is activated if it is located anywhereover the surface of mattress 800 and oriented horizontally. This case issimilar to that of FIG. 8B except that, other factors being equal, themagnitudes of the vertical magnetic field components of coils 820generally will be less than those of coils 810 because of the angularorientations of coils 820 with respect to the horizontal.

FIG. 8D illustrates an alternative configuration in which horizontalprimary coil 830A is positioned between two vertical primary coils 830Band 830C. In this exemplary configuration, coil 830A is embedded inmattress 800 whereas coils 830B and 830C adjoin opposing sides 801 and802 of mattress 800, respectively. Coils 830B and 830B may be containedwithin side boards of a bed including mattress 800, for example. Primarycoil 830A thus is capable of generating vertically oriented magneticfields, and primary coils 830B and 830C are capable of generatinghorizontally oriented magnetic fields, across all of top surface 805.Secondary coil 230 thus may be activated irrespective of its position ororientation above mattress 800, provided that it is not too far removedfrom mattress 800. As is evident, the operational range of secondarycoil 230 above top surface 805 depends on the strength of the magneticfields generated by primary coils 830 and by the height above topsurface 805 of primary coils 830B and 830C.

It will be understood, based on the description above with respect toFIGS. 4A-4D, that any of the primary coils represented in FIGS. 8A-8Dmay be replaced with magnetically equivalent groups of smaller primarycoils. For example, primary coil 830A may have a perimeter that isapproximately co-extensive with the perimeter of top surface 805 ofmattress 800. It may be replaced with a functionally equivalent array ofsmaller primary coils arranged so that the exterior loop segments of thearray also are generally co-extensive with the perimeter of top surface805. Thus, in one of many possible configurations, primary coil 830A mayhave a shape similar to that of primary coil 420 of FIG. 4B, and it maybe functionally replaced with an array of four smaller or eight evensmaller primary coils arranged and configured as shown in FIG. 4A withrespect to primary coils 410.

Yet another type of configuration that may be used to achieve fullcoverage is shown illustratively in FIGS. 9A and 9B. FIG. 9A is aschematic cross-sectional top view of three horizontally disposedprimary coils 910A-C (coils 910). FIG. 9B is a schematic cross-sectionalside view of coils 910 along cross-section line 9A—9A. Two of the coils,910A and 910B, are in approximately the same plane, as indicated in FIG.9B. The third coil, 910C, is in a plane that is approximately parallelto, and below (or, in an alternative implementation, above), the planeof coils 910A and 910B. As seen from the top-view perspective of FIG.9A, an overlap area 921 is defined by the vertical projection of coil910C on coil 910A, and a similar overlap area 922 is defined by thevertical projection of coil 910C on coil 910B. Also, an inter-coil area920 is shown that is between coils 910A and 910B, but within theprojection of coil 910C.

As noted with respect to FIG. 6B, a horizontally disposed secondary coil230 positioned between two first primary coils in the same plane may beactivated by energizing both first primary coils using current flowingin the same direction. Alternatively, such a disposed secondary coil maybe activated by energizing another primary coil that is positionedbetween the first primary coils in another plane. For example, ifsecondary coil 230 is located in inter-coil area 920 between coils 910Aand 910B, it may be activated either by energizing coils 910A and 910Bwith current flowing in the same direction, or by energizing coil 910C.Because secondary coil 230 is, in this example, positioned above theinterior of coil 910C, it will be activated by a vertical magnetic fieldgenerated by coil 910C above its interior.

Importantly, the configuration of FIGS. 9A and 9B may also be used toachieve full coverage with respect to secondary coil 230 in a verticalorientation. As noted, a vertically disposed secondary coil 230generally positioned between two primary coils, or above exterior loopsegments, may be activated by energizing both primary coils usingcurrent flowing in opposite directions. However, as also noted,vertically oriented secondary coil 230 positioned above the middle of aprimary coil generally may not be activated by that coil. Nonetheless,such a positioned and oriented secondary coil may be activated by aprimary coil generally positioned so that one of its exterior loopsegments is aligned with the middle of the other primary coil. Forexample, primary coil 910C, having exterior loop segments 911 and 912,may activate secondary coil 230 if it is vertically oriented andpositioned in positions 930B or 930C, respectively. These positions aregenerally aligned with exterior loop segments 911 and 912, and thus aweaker, horizontally oriented, magnetic field is produced through thosepositions in the manner shown by arrows 767 and 768. As is evident, themagnitude of this magnetic field generally decreases with distance fromthe energized loop segment; thus, the magnetic field atregions/positions 930A or 930D generally will be weaker than those at930B or 930C. In addition to the previously noted steps of increasingcurrent or number of loops, this potential deficiency in fill coveragemay be avoided by providing interlaced arrays of primary coils in two ormore planes.

FIG. 10 illustrates one example of interlaced arrays of primary coils.In addition to addressing potential deficiencies in full coverage, sucharrays may have an additional benefit of reducing the amount of powerrequired by EFS 100. That is, less power generally is required toactivate a secondary coil by generating a magnetic field from a smallprimary coil close to the secondary coil than from a more distantprimary coil or from a large primary coil covering a large area. Arraysof smaller primary coils provide that one or more of the primary coilswill be close to the secondary coil, and thus less power generally willbe required than if more distant, or larger, primary coils wereemployed. In some applications, a similar benefit is derived from thefact that the magnetic field generated by a small primary coil of anarray is more localized than the magnetic field generated by a largeprimary coil. For example, the generation of large-area magnetic fieldsmay interfere with medical equipment, or adversely interact withmetallic objects in or around the recipient.

It will be understood that although the configurations of FIGS. 9A, 9B,and 10 are described herein for convenience as having primary coilsdisposed in two planes (which may include additional planes inalternative configurations), the coils in those planes may, in practice,be very close together. They may be so close as to essentially provide asingle plane with interwoven primary coils. For example, FIG. 10 is aschematic cross-sectional top view that shows what is referred to as twoparallel planes of primary coils. A first plane, shown in solid lines,includes representative coils 1010A-D of an array of first-plane primaryto coils 1010. A second plane, referred to as including an array ofsecond-plane primary coils, is shown in phantom outline. Coils 1020A-Dare representative of this array of second-plane primary coils 1020. Inone of many possible implementations, coils 1010 and 1020 may consist ofwires woven into, or positioned within, a blanket such that the loopsegments of coils 1010 cross under, and may touch (if electricallyinsulated), loop segments of coils 1020 where they cross. For clarityand convenience of illustration, coils 1010 and 1020 are schematicallyrepresented by squares that are disposed within a horizontally orientedmattress 1000. Lead wires into and out of the coils are not shown forclarity.

As is evident from FIG. 10 and the preceding description relating toFIGS. 9A and 9B, coils 1010 and 1020 provide full coverage over thesurface of mattress 1000. (As also is evident, coverage at the perimeterof mattress 1000 may be achieved by the placement of primary coilsadjacent to this perimeter, which are also omitted in FIG. 10 forclarity.) Full coverage using two planes of primary coils has alreadybeen described with respect to FIGS. 9A and 9B. In that description,however, it was noted that a vertically oriented secondary coil 230positioned as in positions 930A or 930D, ie., at some distance from theexternal loop segment of the closest primary coil, may not be activatedunless current is increased, or another measure is taken, to increasethe magnetic field generated by that external loop segment. The arraysof primary coils 1010 and 1020 provide an alternative way of providingthat secondary coil 230, in such orientation and position, will beactivated.

For example, it is assumed that secondary coil 230 is verticallyoriented and positioned in the middle of first-plane primary coil 1010D,as schematically represented in FIG. 10. Secondary coil 230 may beactivated by energizing any one, or any combination, of second-planeprimary coils 1020A-D. Similarly, if secondary coil 230 is verticallyoriented and positioned approximately in the middle of second-planeprimary coil 1020A, it may be activated by energizing any one, or anycombination, of first-plane primary coils 1010A-D. It should beunderstood that primary coils 1010A-D may be at other orientations, suchas at a 45° orientation. As is evident from FIG. 10, the size of primarycoils 1010 and 1020 generally may be determined by the size of secondarycoil 230, among other factors.

Alternative, but not exhaustive, ways of providing full coverage havenow been described. These alternatives depend on various configurationsof primary coils, the selection of one or more primary coils closest tothe secondary coil, the orientation of the secondary coil with respectto the plane or planes of the primary coils, and the strength of themagnetic fields based on various factors such as current strength andthe direction of the magnetic field by proper choice of currentdirections through the coils. Now to be described are the operations ofEFS 100 with respect to determining which of the primary coils areclosest, the orientation of the secondary coil, and the amount ofcurrent to provide to the closest primary coils.

III. Controller

As noted, controller 120 selectively provides current to one or moreprimary coils based on their proximity, and/or orientation with respect,to the secondary coil. For convenience, controller 120 is hereafterequivalently described as selectively providing current to one or moreprimary coils based on their “position” with respect to the secondarycoil. As shown in FIG. 1, controller 120 includes current director 124,proximity detector 126, and orientation detector 122, the operations ofwhich are now described.

A. Current Director

Current director 124 selectively directs time-varying currents throughthe closest primary coils. Current director 124 operates in cooperationwith TET primary circuits 140 and power supply 130. Specifically, TETprimary circuits 140, powered by power supply 130, fashion alternatingcurrents suitable for energizing primary coils in a TET. TET primarycircuits 140 may include any of a variety of known circuits forperforming such a function, or may include circuits to be developed inthe future. The functions of current director 124 are shown in greaterdetail in FIG. 3.

1. Power Adjuster

Current director 124 includes power adjuster 310 that adjusts the amountof current directed to the closest primary coils based on a number offactors. Any of a variety of known techniques may be used to effectuatesuch adjustment; for example, any known amplifier circuit may beemployed.

One factor in determining the amount of adjustment is the proximity ofthe secondary coil to the closest primary coils. In one exemplaryembodiment, a default value for the amount of current may be assumed bypower adjuster 310. This default value may, for example, be a minimumvalue that will activate secondary coil 230 if it is located at adistance from the plane of the primary coils within an average range.This value may be determined empirically, by computation, or by acombination thereof. It is assumed for illustrative purposes that theanterior-posterior dimension of an average person ranges from nine totwelve inches in the area of the abdomen where a secondary coil may beimplanted for operating a total artificial heart. If the primary coilsare implanted two inches below the top surface of a mattress on whichthe recipient reclines, the default value may be such that secondarycoil 230 will be activated if it is located 14 inches from the closestprimary coils. If proximity detector 126 detects that secondary coil 230may be further from the closest primary coils than 14 inches in thisillustrative example, power adjuster 310 may increase the amount ofcurrent so that the magnetic fields generated by the closest primarycoils nonetheless will be sufficient to activate secondary coil 230.

The amount of current required to activate secondary coil 230 alsodepends, as noted, on the size, shape, and number of loops of theclosest primary coils. In the illustrated embodiment, the number ofloops in each primary coil is predetermined in accordance withcalculations based on equation number one, above, using an assumed sizeof secondary coil 230. As will be evident to those skilled in therelevant art, many other known equations and techniques may be used todetermine a default, or other, current value.

To provide one example with respect to the illustrated embodiment, it isassumed that secondary coil 230 is circular with a diameter of threeinches. It may be determined empirically, in view of the requirements ofa typical total artificial heart 215 and battery 220, that the requiredmagnetic field for effective coupling to this secondary coil 230 isapproximately 10³¹ ⁴ weber/meter². To generate 10⁻⁴ weber/meter², acombination of N=3 turns, i=4 amperes, and r=0.05 meter (4″ diameter) isadequate in typical circumstances in which secondary coil 230 is closelycoupled to the closest primary coil. Assuming that secondary coil 230may be on the order of 12 inches away from secondary coil 230, thenumber of primary turns may be increased to maintain the same fieldstrength without an increase in the current used. A combination of N=9turns, i=4 amperes, and r=0.15 meter (12″ diameter or square edge) istypically adequate to activate secondary coil 230 under these assumedcircumstances. It will be understood that these calculations areillustrative only, and that many other solutions are possible underalternative assumptions.

As noted, the orientation of secondary coil 230 may indicate that thecurrent to the closest primary coils should be increased (or,alternatively, that the number of loops in each primary coil beincreased to take orientation into account). For example, if secondarycoil 230 is determined to be predominantly perpendicular to the plane orplanes of the primary coils, and located above an external loop segmentof the closest primary coil (e.g. position 730A of FIG. 7) rather thanbetween closest primary coils (e.g. position 730C), then power adjuster310 may increase current in order to increase the strength of themagnetic field generated by the closest primary coil.

Further, as noted, a typical distance from implanted secondary coil 230to the surface of a mattress may be greater for a recipient lying on hisor her side than if the recipient were in lying on his or her stomach orback. Also, the recipient may not be lying directly on the mattress, butmay be reclining on pillows, sitting up, or be otherwise disposed awayfrom the mattress. In the illustrated embodiment, proximity detector 126detects such greater-than-normal distance, and power adjuster 310therefore increases the amount of current. In other embodiments, aworst-case situation may be assumed such that it is not necessary toadjust the amount of current to the closest primary coils. That is, thedefault value is great enough to ensure that secondary coil 230 will beactivated within any foreseeable range of distances from the primarycoils. In such embodiments, power adjuster 310 need not be included.

2. Current Direction Determiner

Current director 124 also includes current direction determiner 320 thatdetermines the directions of current supplied to two or more closestprimary coils. In some instances, there may be only one closest primarycoil, in which case current direction. determiner 320 need not beemployed. For example, secondary coil 230 may be positioned above themiddle of a primary coil and oriented in the same plane as that coil(e.g., it is positioned in position 640B of FIG. 6). Thus, only thatprimary coil need be energized and direction of current flow isimmaterial with respect to alternating current through a single coil. Inother instances, however, it may be required to energize two or moreprimary coils.

For example, secondary coil 230 may be positioned between two closestprimary coils in approximately the same plane as those coils (e.g., itis positioned in position 640A of FIG. 6). To activate secondary coil230 in this position, the two closest primary coils are energized, withcurrent flowing in the same direction (in phase). In anotherillustrative example, secondary coil 230 may be approximately above themiddle of the square pattern made by primary coils 410 of FIG. 4A; thatis, at location 418 between these four coils. If all four of coils 410are energized by current flowing in the same direction, then, as noted,coils 410 are magnetically approximately equivalent to coil 420 of FIG.4B. Thus, secondary coil 230 is activated by the energizing of coils 410because such energizing is the approximate equivalent of energizing acoil having a center below, and in the same plane as, the secondarycoil.

Current direction determiner 320 thus analyzes information fromproximity detector 126 and orientation detector 122 to determine therelative directions of current flow in the two or more closest primarycoils. Such analysis may be carried out in accordance with any of avariety of known techniques. For example, firmware includinganalog-to-digital converters and logical elements may be employed;appropriate software may be executed by a microprocessor (not shown);analog electrical comparator circuits may be employed, and so on.

3. Primary Coil Selector

Also included in current director 124 is primary coil selector 330 thatselects primary coils to receive current. In the illustrated embodiment,this selection is made based on information provided by proximitydetector 126 and orientation detector 122, as described below.Specifically, one or more primary coils are selected based on thedetected location and orientation of secondary coil 230 and inaccordance with the characteristics of magnetic fields described above.Typically, such selection is made in cooperation with the determinationsmade by current direction determiner 320, and in accordance with knowntechniques as noted with respect to current direction determiner 320.

Primary coil selector 330 also typically employs any of a variety ofknown switching techniques and devices to direct current to the selectedprimary coils. For example, any of a large number of solid-stateelectronic switches, or mechanical switches, may be employed. In FIG. 3,this switching function is represented by the connection of primary coilselector 330 to connectors 102-108. As shown in FIG. 1, connectors 102and 104 are respectively connected to the output and input terminals ofprimary coil 110A, and connectors 106 and 108 are similarly connected toprimary coil 110B.

B. Proximity Detector

As noted, proximity detector 126 determines the one or more closestprimary coils to secondary coil 230, and may also determine the locationof secondary coil 230 with respect to the closest primary coils (ie.,between two coils, over the middle of one coil, over an external segmentof one coil, and so on). For clarity and convenience, such informationhereafter is simply referred to as the “proximity” or “closeness” ofsecondary coil 230. As is evident, the proximity of secondary coil 230to various primary coils typically changes over time as recipient 205moves. Thus, the identification of the closest primary coils mayconsequently change over time. It will thus be understood that thedeterminations made by proximity detector 126 (and by orientationdetector 122, described below) typically are made at intervals, or theymay be made essentially continuously. For example, if calculations andcomparisons are made in an analog system, proximity detector 126 mayprocess information continuously. If calculations and comparisons aremade in a digital system using firmware, software, or a combinationthereof, a period for making calculations may be determined inaccordance with any of a variety of known techniques and in accordancewith the time needed to execute the firmware or software.

In one embodiment, proximity detector 126 employs well known principlesof mutual inductance to determine the proximity of secondary coil 230.Specifically, the inductances of each primary coil at a selectedfrequency, referred to as the “primary resonance frequency,” aremeasured. This frequency may, but need not be, different than thefrequency at which the alternating current changes direction (the“secondary resonance frequency”). The proximity of the secondary coilresults in a mutual inductance that either adds to, or subtracts from,the inductance of the primary coil depending on the winding sense of thesecondary compared to the primary. This change in the inductance of theprimary coil being measured is referred to herein as a “frequency shift”because it changes the primary resonance frequency. As will be evidentto those of ordinary skill in the relevant art, the mutual inductance ofclosely coupled primary and secondary coils is approximated by thefollowing equation: $\begin{matrix}{\frac{M}{L} = {\frac{N_{2}}{N_{1}}\left( \frac{r_{2}}{r_{1}} \right)^{2}}} & (2)\end{matrix}$

Where, M is the mutual inductance, L is the inductance of the primarycoil without the secondary coil being present, N₂ and N₁ are the numberof turns of the secondary and the primary coil, and r₂ and r₁ are thecharacteristic dimension of the secondary and primary coils,respectively. For typical values, such as provided in the illustrativeexample provided above with respect to the operation of power adjuster310, N₂ equals 18 turns, N₁ equals 9 turns, r₂ equals 1.5 inches, and r₁equals six inches. Thus, the ratio M/L is approximately equal to 0.125,or about 12 percent. That is, resonance frequency shifts of as much asapproximately six percent in each direction, depending on the relativedirections of windings, may be expected if secondary coil 230 isproximate to the primary coil being measured.

By thus measuring the proximity of each primary coil, the closestprimary coils are determined. Similarly, it may be determined that aparticular primary coil is the closest coil, but that the resonanceshift is less than would be expected if the secondary coil werepositioned at a nominal distance from the middle of the coil. Suchreduced shift may be due to the secondary coil being between themeasured primary coil and an adjacent primary coil, or at an edge of themeasured coil nearer to the adjacent coil. Measurement of the frequencyshift of the adjacent primary coil may confirm this.

Otherwise (i.e., if the proximity of the secondary coil to the adjacentprimary coil is not as great as would be expected if the secondary coilwere between the measured coil and the adjacent coil, or closer to theadjacent coil), the secondary coil may be at a greater-than-nominaldistance above the measured primary coil. As will be evident to thoseskilled in the relevant arts, frequency shift information from theclosest and adjacent primary coils may thus be used to determine theproximity and the location of the secondary coil.

Proximity detector 126 in this manner determines the proximity of eachprimary coil until one or more closest primary coils are identified or,alternatively, the mutual inductance of each primary coil is determined.One reason that it may be advantageous to measure all primary coils,even if it appears that one or more closest primary coils have beendetected because of a significant frequency shift, is to avoid anerroneous determination due to the placement of a wire or othermagnetically active material on or near mattress 210. Thus, for example,the distribution of inductance due to an electrical chord placed on themattress typically will be different than the distribution ofinductances due to the proximity of a secondary coil of the typetypically implanted in recipient 205.

If all primary coils are measured and proximity detector 126 does notdetect the proximity of secondary coil 230, then the measurement processmay be repeated with more power provided to each primary coil.Optionally, if one or more closest primary coils are not detected, analarm may be enabled in accordance with known techniques to indicatethat the secondary coil is not being activated.

C. Orientation Detector 122

As previously described, orientation detector 122 determines whethersecondary coil 230 is disposed in an orientation predominantlyperpendicular, or alternatively, parallel, to the plane or planes of theprimary coils. Such orientation is measured in the illustratedembodiment by detecting relative changes in the resonance shifts ofadjacent primary coils.

For example, it is assumed that the primary coils are arranged in twoparallel layers in accordance with the example of FIG. 10. It is alsoassumed that the frequency shift due to mutual inductance, as measuredwith respect to second-layer coil 1020A, is relatively high, and thatthe frequency shift with respect to first-layer coils 1010A and 1010Bare approximately the same, and are relatively low as compared to thatof coil 1020A. Thus, for the same reasons discussed above with respectto FIGS. 6B and 7B regarding the characteristics of magnetic fields, itis determined that secondary coil 230 is predominantly perpendicularwith respect to the planes of coils 1010 and 1020, while being locatedat the edge of coil 1020A and equally divided between first-layer coils1010A and 1010B.

As the differences decrease between the frequency shift of coil 1020A,on the one hand, and coils 1010A and 1010B, on the other hand, it may bedetermined that the predominance of the perpendicular orientation isdecreasing; i.e., secondary coil 230 is becoming less perpendicular, andmore parallel, to the planes of coils 1010 and 1020. If the relationshipis reversed, such that the frequency shift of coil 1020A is relativelylow in comparison to that of coils 1010A and 1010B, then it may bedetermined that secondary coil 230 is predominantly parallel to theplanes of coils 1010 and 1020. Orientation detector 122 thus determinesthe orientation of secondary coil 230, within the range frompredominantly perpendicular to predominantly parallel, with respect tothe planes of the primary coils.

Having now described one embodiment of the present invention, it shouldbe apparent to those skilled in the relevant art that the foregoing isillustrative only and not limiting, having been presented by way ofexample only. Many other schemes for distributing functions among thevarious functional elements of the illustrated embodiment are possiblein accordance with the present invention. The functions of any elementmay be carried out in various ways in alternative embodiments.

For example, numerous variations are contemplated in accordance with thepresent invention to carry out the functions of controller 120. Theoperations of orientation detector 122, or portions thereof, may becombined with those of proximity detector 126, or portions thereof.Similarly, either or both of detectors 122 and 126 may carry out theoperations of current director 124, or vice versa. Either, or both, ofthe detectors 122 and 126 may be implemented in alternative embodimentsby mechanical, optical, electromagnetic transmission (e.g., implanting aradio-frequency transmitter with secondary coil 230 and detecting thestrength and direction of its signal), or other detectors. Suchdetectors may be employed with, or instead of, the resonance frequencyshift detectors described with respect to the illustrated embodiment.

Also, the division of functions of current director 124 into poweradjuster 310, current direction determiner 320, and primary coilselector 330 is made for convenience of illustration and otherdistributions of these, and other, functions may be made in alternativeembodiments. Also, as noted, many possible variations are possible withrespect to the shape of primary coils, the number of loops in them,their orientations in planes with respect to each other and theirhousings, patterns in which arrays of primary coils are arranged, andother factors.

In addition, it will be understood by those skilled in the relevant artthat control and data flows between and among functional elements of theinvention may vary in many ways from the control and data flowsdescribed above. More particularly, intermediary functional elements(not shown) may direct control or data flows; the functions of variouselements may be combined, divided, or otherwise rearranged to allowparallel processing or for other reasons; data structures may beemployed to store and/or manipulate data; the sequencing of functions orportions of functions generally may be altered; and so on. Numerousother embodiments, and modifications thereof, are contemplated asfalling within the scope of the present invention as defined by appendedclaims and equivalents thereto.

What is claimed is:
 1. A transcutaneous energy transmission (TET)device, comprising: at least one secondary coil disposable within apatient and adapted to receive transcutaneous energy transmission; aplurality of primary coils, each adapted to be placed outside a patientand constructed and arranged to carry a time-varying current to transmittranscutaneous energy to the secondary coil; and a controllerconstructed and arranged to detect the position of the secondary coilrelative to each of the plurality of primary coils and to selectivelyprovide current to one or more of the plurality of primary coils basedon the detected position of each primary coil with respect to thesecondary coil.
 2. The TET device of claim 1, wherein the controllercomprises: a proximity detector constructed and arranged to identify aquantity of the plurality of primary coils that are closest to thesecondary coil; and a current director, responsive to the proximitydetector and electrically coupled to the plurality of primary coils,constructed and arranged to selectively direct time-varying currentsthrough the closest primary coils.
 3. The TET device of claim 2,wherein: the proximity detector identifies the quantity of closestprimary coils utilizing a pressure sensor.
 4. The TET device of claim 2,wherein the proximity detector comprises a resonance frequency shiftdetector that detects a shift in inductance of one or more primary coilsdue to a proximity of the secondary coil.
 5. The TET device of claim 1,wherein current director directs time-varying currents to flow throughthe closest primary coils so that each current flows in the samedirection when the secondary coil is disposed in a plane predominantlyparallel to the planes of the closest primary coils.
 6. The TET deviceof claim 1, wherein current director directs time-varying currents toflow through the closest primary coils so that a current in each closestprimary coil flows in a direction opposite to a direction of a currentin an adjacent closest primary coil when the secondary coil is disposedin a plane predominantly perpendicular to the planes of the closestprimary coils.
 7. The TET device of claim 2, wherein the quantity of theclosest primary coils identified is a predetermined number.
 8. The TETdevice of claim 2, wherein the quantity of the closest primary coilsidentified is based on a size of the secondary coil.
 9. The TET deviceof claim 2, wherein the proximity detector comprises an optical sensor.10. The TET device of claim 2, wherein the proximity detector comprisesa mechanical sensor.
 11. The TET device of claim 2, wherein theproximity detector comprises an electromagnetic transmission detector.12. The TET device of claim 2, wherein the quantity of the closest coilsis two.
 13. The TET device of claim 2, wherein the proximity detectordetermines an approximate distance between one or more of the closestprimary coils and the secondary coil, and the current director increasesthe currents through the closest primary coils when the proximitydetector determines that the distance is greater than a nominalthreshold value.
 14. The TET device of claim 2, wherein the currentdirector includes a power adjuster for adjusting the amount of currentdirected to the closest primary coils.
 15. The TET device of claim 2,wherein the current director includes a current direction determiner fordetermining the directions of current supplied to two or more closestprimary coils.
 16. The TET device of claim 2, wherein the currentdirector means includes a primary coil selector for selecting primarycoils to receive the current.
 17. The TET device of claim 1, whereineach of said plurality of primary coils comprises a phase and a sizeadapted to optimize coupling with the secondary coil.
 18. The TET deviceof claim 1, wherein each of the primary coils is disposed in a planesubstantially parallel to, including the same plane as, a plane of eachof the other primary coils.
 19. The TET device of claim 1, wherein theprimary coils are disposed in two or more substantially parallel planes,including: a first plane having one or more first-plane primary coilsthat, when energized, generate electromagnetic fields having one or moredead zones; and a second plane having at least one second-plane primarycoil positioned with respect to the one or more first-plane primarycoils to generate, when energized, at least one electromagnetic fieldencompassing the one or more dead zones.
 20. The TET device of claim 1,wherein the primary coils are disposed in two or more substantiallyparallel planes, including: a first plane having two or more mutuallyadjacent first-plane primary coils; and a second plane having at leastone second-plane primary coil positioned with respect to the two or moremutually adjacent first-plane primary coils so that the projection of amagnetic center of the second-plane primary coil on the first plane isapproximately equidistant from magnetic centers of each of the two ormore mutually adjacent first-plane primary coils.
 21. The TET device ofclaim 1, wherein the primary coils are disposed in two or moresubstantially parallel planes, including: a first plane having fourmutually adjacent primary coils positioned with respect to each other ina roughly square arrangement; and a second plane having one primary coilincluding a second plane primary coil geometric center, the second planeprimary coil positioned so that the projection of the second planeprimary coil geometric center on the first plane is approximatelycentrally located among the four first plane primary coils.
 22. The TETdevice of claim 1, further comprising one or more articles of furniturehousing the plurality of primary coils.
 23. The TET device of claim 1,further comprising a cardiac assist device electrically coupled to thesecondary coil.
 24. A cardiac-assist device comprising: a pumping means;and a transcutaneous energy transmission (TET) device electricallycoupled to the pumping means, comprising: a secondary coil implanted ina subject and adapted to receive transcutaneous energy transmission; aplurality of primary coils, each adapted to be placed outside a subjectand constructed and arranged to carry a time-varying current to transmittranscutaneous energy to the secondary coil; and a controllerconstructed and arranged to detect the position of the secondary coilrelative to each of the plurality of primary coils and to selectivelyprovide current to one or more of the plurality of primary coils basedon the detected position of each primary coil with respect to thesecondary coil.
 25. The cardiac-assist device of claim 24, wherein: thepumping means includes an artificial heart.
 26. The cardiac-assistdevice of claim 24, wherein: the pumping means includes aventricular-assist device.
 27. The cardiac-assist device of claim 24,wherein the controller comprises: a proximity detector constructed andarranged to identify a quantity of the plurality of primary coils thatare closest to the secondary coil; and a current director, responsive tothe proximity detector and electrically coupled to the plurality ofprimary coils, constructed and arranged to selectively directtime-varying currents through the closest primary coils.
 28. Thecardiac-assist device of claim 24, wherein a phase and size of saidplurality of primary coils is selected to optimize coupling with thesecondary coil.